Variation in soldier investment is linked to the evolution of termite soldier defense strategies

The evolutionary success of social insects is attributable to their division of labor, often facilitated by specialized castes. One such caste, the soldier, specializes in colony defense. In termites, soldier defense strategies vary across taxa, ranging from modes relying on nest architecture (strong-point strategy) to those employing active engagement with intruders (counter-attack strategy). While previous studies have suggested that soldier morphology and behavior are associated with colony defense strategy, the broader ecological and evolutionary factors shaping variation in defense strategies remain unexplored. In this study, we use data from the literature to examine the relationship between defense strategy, soldier proportion, and nesting strategy within a phylogenetic comparative framework. Our results reveal a clear pattern: species employing strong-point strategies exhibit lower soldier proportions, whereas counter-attack strategists typically invest more heavily in soldiers. Phylogeny-based inference of trait evolutionary history indicated that counter-attack strategies evolved independently from strong-point strategies at least 12 times, while no evidence was found for the reverse pattern. We find that factors such as foraging type (non-foraging vs. foraging), defense behavior (e.g., autothysis, phragmosis), and life-history traits can, at least partly, explain interspecific variation in soldier investment. Notably, foraging strategy was significantly associated with the evolution of defense strategy, with non-foraging termites largely restricted to strong-point strategies. Overall, our findings underscore the multifactorial nature of termite defense strategy evolution.